Often, layers of differing compositions must be applied to a substrate. It is common to apply a primer coating under a paint to improve the anchorage. In the manufacture of photographic film, as many as twelve layers of differing compositions must be applied in a distinct layered relationship with close tolerances on uniformity. The use of sequential coating operations can produce a plurality of distinct superposed layers on a substrate. However, this is costly and time consuming and may require a large investment in the sequential coating and drying stations.
Methods of applying simultaneous, multilayer coatings are discussed in the book: Cohen, E. D. and Gutoff, E. B., Editors, 1992, Modern Coating and Drying Technology, chapter 4, VCH Publishers, New York. Slot or extrusion, premetered die coaters are disclosed in U.S. Pat. Nos. 2,761,419 and 2,761,791 and many improvements have been developed over the years. With these coaters, the surface of the web to be coated is brought into contact with or in close proximity to the die and a plurality of superposed layers is deposited. Each coating composition is metered to the coating die which deposits them as layers on the web. However, the uniformity of the gap to the web limits the quality of the coatings, and the maximum speed of operation is limited.
Another method of simultaneous, multilayer coating is curtain coating. U.S. Pat. No. 3,508,947 teaches the use of this method with the coating of photographic elements. Curtain coating uses a free falling vertical curtain of liquid which impinges upon the web traversing the coating station. This patent teaches a method of forming the curtain from a plurality of distinct layers to accomplish a multilayer coating on the web. The gap between the coating die and the web is much greater than previous methods and the speeds of application are substantially greater. However, this method has set caliper and speed limitations.
A limitation of curtain coating is that for any formulation there is a minimum flow rate below which a stable curtain can not be maintained. This prevents coating thinly at slow and moderate speeds. Since the slide and the curtain simultaneous multilayer methods were first introduced, many refinements have been invented. However, there is still the need for improved low speed and high speed simultaneous layer method of coating.
The technology of single layer air knife coating is summarized in Chapter II of the book Pulp and Paper Manufacture, Volume 8; Coating, Converting, and Specialty Processes, Michael Kouris, Technical Editor, 3rd edition, 1990, published by The Joint Textbook Committee of the Paper Industry, TAPPI and CPPA, Atlanta, Ga. Additional description is in chapter 5 of the book by Cohen and Gutoff. Air knife coating is characterized by the application of an excess of a single coating fluid composition to a web followed by the removal of a portion of this fluid by a gas jet issuing from a nozzle. There is a low speed region of application where low gas pressure is used in the nozzle. Excess coating is forced counter to the web direction of motion and a controlled amount passes through the gas jet on the web surface. This technology has been employed by the photographic industry. There is a high speed region of operation employed by the paper coating industry and in molten metal coating by hot dip steel strip manufacturers. In this case, the gas pressures and web speeds are high and the excess fluid is often atomized by the jet. Both the low and high speed techniques are known only as single layer coating methods using a single coating fluid composition, and they have been practiced for more than fifty years. Both technologies have used coating applicator dies to apply the excess of coating to the substrate before passing the gas jet. These dies are used to crudely apply the excess, and they are used to apply only a single coating fluid composition.
The conventional air knife coating method suffers in range of lo applicability primarily because it coats only one layer at a time, and because it has minimum coating caliper limitations. To produce thin dried coatings, the mass of solids passing through the gas jet per unit of substrate area and left on the substrate must be low. The gas velocity, percent solids, and coating viscosity are the dominant variables controlling this coating weight. Thinner coatings may be obtained by reducing the percent solids, reducing the viscosity, or increasing the jet velocity. There will always be economic and physical limitations on all of these. If the percent solids is reduced, more diluent liquid must be added, increasing both cost and drying time. Reducing the viscosity requires changing the formulation and may result in unwanted flow of the coating after passing the jet and before drying or solidification. Jet velocity increases are limited by numerous practical considerations including the cost and complexity of exceeding the speed of sound with the jet, the mess created by misting the excess coating fluid, and the noise of a high velocity jet.
There is a need for a more versatile multilayer coating method and a multiple layer air knife coater. There is also a need for an improved air knife coater for applying a single dried layer of a coating from a composite layer fluid. And there is a need for a new method which coats thin wet coatings at low speeds (25 microns at 10 m/min web speeds) as well as at high speeds.